fhdl/structure: improved bits_for function

diff --git a/migen/fhdl/structure.py b/migen/fhdl/structure.py
index 16dd435f789b97ce8e112fcd327199ebd855be30..443ddea8eeacade14c8a67191a99eb320d3d1284 100644 (file)
--- a/migen/fhdl/structure.py
+++ b/migen/fhdl/structure.py
@@ -1,31 +1,30 @@
-import math
 import inspect
 import re
 from collections import defaultdict
 
 from migen.fhdl import tracer
 
-def log2_int(n):
+def log2_int(n, need_pow2=True):
        l = 1
        r = 0
        while l < n:
                l *= 2
                r += 1
-       if l == n:
-               return r
-       else:
+       if need_pow2 and l != n:
                raise ValueError("Not a power of 2")
+       return r
 
-def bits_for(n):
+def bits_for(n, require_sign_bit=False):
        if isinstance(n, Constant):
                return len(n)
+       if n > 0:
+               r = log2_int(n + 1, False)
        else:
-               if n < 0:
-                       return bits_for(-n) + 1
-               elif n == 0:
-                       return 1
-               else:
-                       return int(math.ceil(math.log(n+1, 2)))
+               require_sign_bit = True
+               r = log2_int(-n, False)
+       if require_sign_bit:
+               r += 1
+       return r
 
 class BV:
        def __init__(self, width=1, signed=False):